Patch for ticket #1769. Remove the rows in groups_users when the user or the group is deleted.

modules/user/models/user.php

@@ -43,6 +43,12 @@ class User_Model_Core extends ORM implements User_Definition {
     $old = clone $this;
     module::event("user_before_delete", $this);
     parent::delete($id);
+
+    db::build()
+      ->delete("groups_users")
+      ->where("user_id", "=", empty($id) ? $old->id : $id)
+      ->execute();
+
     module::event("user_deleted", $old);
     $this->groups_cache = null;
   }